How far in advance must I prepay at Alamo to receive discount?

[Cameo]

I see that you can prepay at Alamo and get an additional 15% off. How far in advance must you prepay? Also, I think it says you can't use a coupon. I currently have a coupon, but it might be cheaper to take the prepay option and get rid of the coupon. Has anyone done this prepaying and how did it work for you?

 

[HappyLawyer]

anytime up to 24 hours, i played around with my resv for dec 06 and it would take it now,

 

[mochabean]

I think you have to use PREPAY as the corp ID when reserving in order for it to process the transaction. For my rental, the difference between using PREPAY and a coupon with alternate corp ID discount was about $5, so I figured it wasn't worth it.
I don't know if you can reserve using a different corp ID and then calling them to prepay, as the 15% discount only comes up when you use PREPAY as the corp ID. They would probably have to change your corp ID and you would lose any discounts associated with that corp ID.